1. Field of the Invention
The present invention relates to a process for selectively removing undesirable peripheral photoresist material (e.g. edge beads) from a photoresist coated-substrate with specific solvent mixtures of ethyl lactate (EL) and methyl ethyl ketone (MEK) which dissolve this peripheral material. In particular, the present invention relates to a process for selectively removing undesirable peripheral material of a photoresist coating comprising a novolak resin and a naphthoquinone diazide photosensitizer from the edge region of an underlying silicon-containing wafer by contacting this peripheral coating material with specific mixtures of ethyl lactate and methyl ethyl ketone which dissolves the peripheral material and allows for its later removal. Furthermore, the present invention relates to, as novel articles of manufacture, the particular mixtures of ethyl lactate and methyl ethyl ketone which are useful for this function.
2. Description of the Prior Art
In the production of micro integrated circuit components or "chips" it is customary to coat a wafer or substrate with a photosensitive material (generally called a photoresist) and then imagewise expose the coated substrate to a light source through a pattern or mask. The portions of the photoresist coating exposed to the light source become chemically altered to permit selective removal of either the exposed portion of the coating or the unexposed portion by a developing solution.
The resultant selectively exposed and developed substrate is then subjected to further processing steps, such as etching, diffusion, plating or the like, to provide the layers of microcircuitry which will eventually result in the production of a number of integrated circuits or chips on the substrate.
Because of the minute spacing between circuit components and conductor paths, introduction of foreign particles, such as dust or the like, is scrupulously avoided. A single particle of dust on the substrate, for example, may result in the malfunction and, therefore, loss of one of the chips made from the substrate. A number of such foreign particles on a substrate may therefore result in a serious decrease in the yield from the wafer or substrate.
It has been found that one source of these unwanted particles is the unused photosensitive coating present at the periphery of the coated substrate.
In most commercial operations, the photoresist coating material is customarily applied as a liquid to the center of a static or spinning substrate (e.g. silicon wafer or the like) and the substrate is then spun at a preselected speed to cause the coating solution to flow or evenly spread across the surface by centrifugal force. Excess material is ejected from the substrate periphery into a catch cup. The objective is to obtain a coating having a uniform thickness over substantially all of the substrate or wafer surface. However, a slight amount of the coating flows over the outer edge of the substrate and onto the periphery of the backside surface. Thus, such spin coating processes often result in extra accumulations of the coating material on the edges of both the coated surface and the backside surface, as well as forming small beads of coating material on the periphery of the substrate between the coated surface and the backside surface of the substrate. This unwanted peripheral material is not used in lithographic processes for making micro integrated circuit components. It has been surmised that the handling of the substrates during later processing may result in the dislodgment or flaking off of the coating on the periphery of the coated substrate with at least some of the dislodged particles becoming inadvertently scattered over the remainder of the coated substrate resulting in eventual defects.
U.S. Pat. Nos. 4,113,492 (Sato et al); 4,510,176 (Cuthbert et al); 4,518,678 (Allen); and 4,685,975 (Kottman et al) all teach particular processes and apparatus for selectively removing this unwanted peripheral coating material by selectively contacting that material with a solvent capable of dissolving it. All of these four patents are incorporated herein by reference in their entireties. This operation is generally known in the art as "edge bead removal" and various solvents have been used commercially for this operation. See column 6, lines 19-46 of U.S. Pat. No. 4,113,492 which lists several suitable solvents. Before this invention, the preferred solvent for commercial applications was ethylene glycol monoethyl ether acetate (EGMEA). However, this particular solvent raises some toxicological concerns. Aliphatic ketones, such as acetone and methyl ethyl ketone, have also been employed singly for this purpose, but are considered hazardous due to their volatility and odoriferous nature. Accordingly, there is a need in the art for better edge bead removal solvents. The present invention is a solution to that need.